A conventional vehicular heating device is often constituted by a hot water heater which heats a vehicle interior by utilizing a high-temperature engine coolant. Unexamined Japanese Patent Publication No. 10-76837 discloses a vehicular air-conditioning device developed from an existing hot water heater. This vehicular air-conditioning device additionally includes a structure for heating a coolant of a hot water heater by utilizing a heat pump to achieve higher heating performance than the existing hot water heater. According to the vehicular air-conditioning device of Unexamined Japanese Patent Publication No. 10-76837, a coolant for cooling an engine is configured to pass through a condenser, a heater core, and an evaporator in this order as a serial flow. The coolant having passed through these components is again introduced into the engine. The vehicular air-conditioning device of Unexamined Japanese Patent Publication No. 10-76837 further heats the engine coolant at the condenser by utilizing a refrigerant discharged from a compressor to improve heating performance.